The coatings industry is continuallly in need of new coatings systems which will crosslink at moderate temperatures (15.degree.-35.degree. C.; 60.degree.-95.degree. F.) and avoid the use of toxic crosslinking reagents such as isocyanates. The Michael Reaction is a nucleophilic attack by an anion (the Michael donor) on an olefin which bears an electon withdrawing group (this olefin is known as the Michael acceptor). The anion is formed by the removal of an acidic proton from the Michael donor by a basic catalyst. The Michael reaction of acetoacetylated polymers with unsaturated polyesters provides a means of thermosetting these polymers at moderate temperatures. The Michael reaction of acetoacetylated polymers is an especially desirable mode of crosslinking because the final properties of the coating can be tailored to suit individual applications by appropriate selection of the backbone resin which bears the acetoacetyl group. In addition, the Michael reaction provides a desirable means to functionalize acetoacetic acid derivatives without causing the crosslinking of such moieites.
Conventional Michael catalysts, i.e., strong bases such as hydroxide and methoxide, suffer several serious drawbacks. Such strong bases are either insoluble or only marginally soluble in many coatings solvents, and may precipitate and thus cloud the coating as the solvent evaporates. Also, such strong bases may catalyze the Michael reaction too vigorously, which results in too short a pot life for the coatings system. These strong bases also tend to be sensitive to the type of solvent employed, which increases the problems associated with formulation.
Nitrogenous bases are not generally known to catalyze the Michael reaction at low temperatures. Indeed, U.S. Pat. No. 4,408,018 indicates that amine bases such as triethanolamine will not promote crosslinking of acetoacetylated thermoplastic materials into thermoset resins.